Airplane approach control system



Aug. 19, 1947. s. N. wlGHT ErAL AIRPLANE APPROACH CONTROL SYSTEM Filed June 16, 1945 11 sheets-sheet 1 Aug. 19, 1947.

S. N. WIGHT ETAL AIRLANE APPROACH CONTROL YsTEM 11 Sheets-Sheet 2 Filed June 1.6, 1945 .Edi

Aug. 1,9, 1947.

S. N. WlGHT ETAL AIRPLANE APPROACH CONTROL SYSTEM Filed June 16, 1945 11 Sheets-Sheet 3 ESQ N Aug. 19, 1947. s. N. wlGH-r ETAL AIRPLANE APPROACH CONTROL SYSTEM 11 Sheets-Sheet 4 Filed June 16, 1945 I .ad

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S. N. WIGHT ET'AL AIRPLANE APPROACH CONTROL SYSTEM Fiied June 16, 1945- Aug. 19,1947. 2,425,973

11 Sheets-Sheet 6 I l l l I i *il 2" nl l l Q135P* i s l i o T INVENTQR U *g NVvfgh, 0.5.Fneld N @n d N BColey Q g BY :1 5 Ll. THEIR ATTORNEY Aug. 19, 1947. s. N. wlGH'r ETAL AIRPLANE APPROACH CONTROL SYSTEM 11 Sheets-Sheet 7 Filed June 16, 1945 Aug., 19, 1947.

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s. N. wlGHT ET AL AIRPLANE APPROACH CONTROLv SYSTEM Aug. 19, 1947.A

1l 'Sheets-Sheet 9 Filed June 16, 1945 Aug. 19, 1947. s, N, wlGHT Ef AL 2,425,973

AIRPLANE APPROACH CONTRL SYSTEM Filed June 16, 1945 11 Shee'ts-Sheet 10 Landing Strips LF FIG-O. X

|- n n u n THEIR ATTORNEY s. N. wlGHT ETAL AIRPLANE APPROACH CONTROL SYSTEM Aug. 19, 1947.

Filed June 16, 1945 11 Sheets-Sheet 11 N w X N H.. X

INVENTORS. 5N Wight, O5 Field THEIR A'TTORNEY Patented ng. i9, i947 'iE S 34 Claims.

rihis invention relates to a control and indication system. for indicating traffic conditions on various air routes and air route portions.

At the approach to airports it is often found necessary to store airplanes in one or more holding stacks to await their time to land. This cornes about by reason of the fact that airplanes arrive over numerous altitudes over numerous traiiic lanes all of which airplanes must in most instances be landed on a single landing strip one at a time and not spaced too closely.

In View of the foregoing and other considerations itis proposed in accordancev with the present invention to preferably provide a plurality of like constructed control and indication boards one for each of one or more operators, each board oi which is provided with controllable devices, token jacks and indicating lights, each board depicting the same route, route portions `and holding stacks and to so interconnect the devices on these boards that each board will indicate tralic conditions at all points but will distinctively indicate on each board the airplanes that are to be dispatched by the operator at that board.

lt is further proposed as an object of the present invention to provide means to enable a. particular operator to pre-empt a portion of a route has been pre-empted or occupied hy operator oi another board, or in combination with a token may indicate that the route or Stack portion is occupied by an airplane under the control of' the operator oi that board. It is also proposed to accomplish such pre-empting' through the rnediurn of a self-restoring manually operable device such as a push buttonV and means having a stick feature so that special manipulation will be' reor an altitude in a holding stack for his own use which no other operator may thereafter use until the first mentioned operators right thereto has been cancelled.

As another object of the present invention it is proposed to provide means whereby if a plurality of operators try to either pre-empt a route or stack portion, or try to hold such route or stack portion, only one of such plurality of operators will obtain rights to instruct an airplane to occupy such route or stack portion.

Another object of the present invention resides in the employment of mag-stick or similar staywhere-put relays to aid in the indication of traiiic conditions so that a temporary failure of the power source will not cause the stored information to be lost as would be the case if electrical stick relays of the neutral type were used.

Another objectl of the invention resides in the provision of token receptacles or jacks in which tokens identifying airplanes may be inserted and each oi which is provided with contacts to control circuits dependent upon Whether or not there is a token in such token jack.

In accordance with another object of the present invention it is proposed to provide indicating means for each route or stack portion to indicate whether such route or stack portion is clear, has been pre-empted by the operator of that board,

quired toy cancel a. pre-empted airway route portion.

In accordance with another' object of the prsent invention it is proposed to provid'elockirig' means whereby an operator is unable toV preempt a route portion or stack altitudeto'his own use, all other conditions being favorable, unless both his token jack and the corresponding token jack ci his monitor for that route'portion orstack altitude are free oi tokens:

Other objects, purposes and characteristic ieaturesof the invention will be in'part pointed out hereinafter and will in part be obvious from the accompanying drawings in which:

Figs. 1A, 1B and 1C show respectivelythe operators or dispatchers panels or boardsX, J vandZ, the monitors panels (shown inFigs. 21A-2E) are identical except for the omission of all the push buttons and for the omission of indicating lights R from the intermediate Zones WI andEI;

Figs. 2A, 2B, 2C, 2D and-2E show respect'il/elyk the apparatus-on the operators'panel and on'the nionitors panel for stacks and route portions WO, Wl, CS, El and EO, respectively;

Fig. 3 shows another View of a modified operatorsboard X to illustrate Various airway traiic conditions;

Fig. 4 illustrates a ground plan for one arrangement of central stack CSfou'ter stacks WO and EO, and intermediate zones WIand EI;

Figs. 5A and 5B illustrate a front view and a side elevation, respectively, of the tokens employed on the operators and the monitors boards.

Fig. 6 illustrates the interlocking portion of the various push button controlled circuits including the various push buttons PB shown in Figs. 2li-2E but illustrating these circuits only up to and including the operators token'jack contact;

Fig. 7 illustrates the various circuits for con'- trolling the green lamps G on both the op'erators and the monitors board and also shown in Figs. 2li-2E; and l Fig. 8 illustrates howv the circuits of Figs'. 2A-2E are modified where three tokenjacks--per altitude are used as is the case for altitudes l., 2, and 3, as shown in Figs. 1A, 1B and 1C,

The present invention may be used with any suitable form of airplane landing apparatus or procedure but is preferably used in combination with the landing apparatus including a computer as disclosed in the prior application of Field, Wight and Hewes, Ser. No. 573,876, filed January 22, 1945.

Structure-Referring to Fig. 1A it will be seen that the central panel of the operators or dispatchers board depicts the central holding stack CS (see Fig. 4) in which airplanes may be held at various altitudes, ten altitudes i to ii), inclusive, of which have been conventionally depicted on the panel, suitable indicators and apparatus for the ground level also having been illustrated. Similarly, the apparatus for outer stacks WO and EO (see Fig. 4) have been illustrated as have also the apparatuses for intermediate Zones WI and EI. It will be seen that each of these panel portions is provided with both a push button PB and a token jack OTJ for each altitude except that altitudes I, 2, and 3 for the central stack have been provided with three token jacks each. It will also be seen that each altitude for each stack WO, CS and EO has been provided with three indicating lamps G, Y and R whereas the panels for intermediate zones WI and EI have been provided only with indicating lamps G and Y. Three approach token jacks have been illustrated for each altitude of outer stacks WO and EO but these token jacks are not provided with contacts and are merely used as a means for supporting a token at a suitable location, such token having been illustrated in detail in Figs. 5A and 5B. The boards for operators J and Z, illustrated in Figs. 1B and 1C respectively, are identical to the board illustrated in Fig. 1A and for this reason like elements have been designated by like reference characters. In this connection it may be pointed out that each device identied by a reference character will be further characterized by designating the particular panel on which the device is located. Each altitude is designated by its number on a small plate under the heading ALT.

Referring now to Figs. 2A-2E of the drawings, it will be seen that Fig. 2A illustrates the apparatus located on both the dispatchers and the monitors control board associated with the outer stack WO; Fig. 2B shows the apparatus associated with the intermediate zone WI; Fig. 2C illustrates the apparatus associated with the central stack CS; Fig. 2D illustrates the apparatus associated with the intermediate zone EI; and Fig. 2E shows the apparatus associated with the outer stack EO for one altitude of both the dispatchers panels and the monitors panels. The ground layout for these stacks and zones are shown in Fig. 4. As shown, the apparatus associated with the dispatchers or operators panel is illustrated in the upper part in each one of these drawings (Figs. 2A-2E) whereas the apparatus associated with the monitors panel is illustrated in the lower part of each of these drawings. In this connection, it should be noted that, for the purpose of simplifying the circuits illustrated the apparatus and circuits located on boards X, J and Z respectively, for the outer stack WO have been illustrated in one drawing (Fig. 2A) whereas these portions for operators X, J and Z of the operators boards for outer stack WO have been illustrated in different drawings, namely, in the drawings of Figs. 1A, 1B and 1C respectively. Had the apparatus illustrated in the circuit diagrams of Figs 2A-2E been segregated with all of the apparatus for board X on one drawing, all of the apparatus for board J on another drawing, an extremely large number of wires would have been necessary7 to connect these drawings and for this reason the drawings have been simplified as illustrated in Figs. 2A to 2E, inclusive.

It is desirable to use push buttons which are spring returned and which need be only momentarily operated to manifest certain actions by the operator. In order to maintain an indication that such momentary operation has taken place it is necessary to use a suitable hold-over device such as a neutral stick relay or a magstick relay, so that the relay will remain in its last operated position. Since a neutral stick relay is maintained energized through a stick circuit including its own front contact and is biased away from the energized position, any momentary power failure if such neutral relay were used would cause all of such relays to drop to their deenergized position as a result or which any manifestation of previous momentary push-button operation would be lost upon each power failure, for which reason mag-stick relays MS have been provided. These mag-stick relays MS remain in their last actuated position irrespective of power failures.

In order to provide suitable interlocking between different operators boards X, J and Z and between different route portions WO, WI, CS, EI and EO lock relays LR have been provided and these lock relays are controlled in conjunction with the token jacks, push buttons and magstick relays already described. Since the magstick relays MS and the lock relays LR are controlled in like manner on all of the fifteen panel boards illustrated in Figs. 2A to 2E, inclusive, especially insofar as the circuit portion on each of these panels is concerned, these circuit portions will be described for the board X of outer stack WO and like portions of all corresponding circuits on other panels will be assigned like reference characters.

The wires extending from Fig. 2A to Fig. 2B have been assigned reference numbers Il, l2, I3, lit, i5, l5, l'i, IS and i9. The wires extending from Fig. 2B to Fig. 2C have been designated by reference numbers il, il, 2i, 22 23, 2G, 25, 2G, E?, 28, 253, 3l, 32 and '53. The wires extending from Fig. 2C to Fig. 2D have been identied by reference numbers il, 22, 23, 2Q, 2:9, 3i, 32, 33, 35, 3? and 4l. The wires extending from Fig. 2D to Fig. 2E have been identiiied by reference numbers i?, lil, (l2, 43, lili, 45, d, [il and 48.

Referring to board X (Fig. 2A) it will be seen that the lock relay LR is normally energized through a circuit beginning at the mid-point CC of a suitable source of current, and passing through contact 5S of the monitors token MIJ on the monitors board X, stack WO, push button contact 5l on the operators board X of this same outer stack, token contact 52 of the token jack OTJ on the operators panel, contact 53 of the mag-stick relay MS, operators board X, winding of the lock relay LR, board X, outer stack WO, to another terminal of this same source. If now the operator depresses his push button PB, board X, outer stack WO (Fig. 2A), this circuit is broken and a circuit is prepared from the terminal of a suitable source of current through suitable interlocking contacts more specifically described hereinafter (see Fig. 6) and through front contact 55 of the lock relay LR, board Z, outer stack WO (Fig. 2A), front contact 56 of the lock relay LR, board J, outer stack WO, (Fig 2A), contact 53 of the operators token jack OTJ, board X, outer stack WO, back Contact 59 of the lock relay LR, board X, outer stack WO, winding of the relay MS, normally open contact 5l of push button PB, board X, outer stack WO (Fig. 2A), Contact E@ of the associated monitors token jack, to a center tap CC of said source of current. It will be seen that the circuit just traced is open at back contact 5 of the lock relay LR. As just pointed out depression oi the push button PB, board X (Fig. 2A) caused opening of its normally closed contact El and thereby caused deenergization of the lock relay LR for this panel WO of board X. rlhis results in the closure of back contact 59 so that the prepared circuit just traced for the mag-stick relay MS is completed and causes operation of this mag-stick relay MS from its left-hand to its right-hand position. It will be seen that this operation or" the mag-stick relay MS, board X (Fig. 2A) causes the green light C.- on this panel X (Fig. 2A) to be extinguished and the associated yellow light Y to be energized. The green lights G on panels J and Z. (Fig. 2A) are also extinguished but their associated yellow lights are not lighted. Their red lights R will, however, be lighted. It is readily seen that had the operator inserted a token in the token jack OTJ, the lock relay LR would also have been deenergized as would also have been the case had the monitor inserted a token in his token jack MTJ for this outer stack WO, board X (Fig. 2A).

if now the operator and the monitor each insert a token in their token jack for panel WO, board X, all for reasons pointed out hereinafter, a circuit is closed for returning the mag-stick relay MS of this same board back to its normal left-hand position which may be traced as ollows: starting at the terminal of a suitable source of current, contact ti of monitors token Vjack MTJ, contact E8 of the corresponding operators token jack OTJ, panel of outer stack W0, board X (Fig. 2A), assuming its right-hand position because there is presumed to be a token .5:

now inserted in this token jack OTJ, back cntact 59 of the lock relay LR, winding of the magstick MS, normally open contact b2 of mag-stick relay MS, board X (Fig. 2A) to the mid point CC of said source of current. It will be observed that current now flows in the opposite direction through the mag-stick relay MS from that which iiowed therein when the circuit for this magstic'k relay was traced through the normally open Contact l of push button PB. For this reason the mag-stick relay MS is operated from its righthand to its left-hand position to cause the yellow light Y to be extinguished and the red lamp R. to be lighted. The circuit just traced is opened at contact 52 as soon as the relay has been oper-ated beyond its middie position. For reasons already pointed out like elements for the boards J and Z of outer stack 'WO and for boards X, J and Z for each or" the stacks CS and EO and each of the Zones WI and EI will be assigned like reference characters except that lock contacts 55 and 5t will be designated 65 and for circuits associated with board J and will be designated l5 and 'it for circuits associated with board Z.

By referring to the lfifteen panels illustrated in Figs. 2A to 2E, inclusive, it will be seen that each of the push buttons PB has associated therewith a contact 5l which has two active positions that each of the operators token jacks OTJ has associated therewith a normally closed Contact 52 and a double throw contact 58 and that each of the monitors token jacks MTJ, located on the monitors board, is provided with a normally closed contact 5d and a normally open contact 6i. By studying the circuits on the panel X (Fig. 2A) for instance, it is readily seen that the lock relay IR is normally energized signifying normal conditions. It is also readily seen that with the mag-stick relay MS to the left signies normal conditions and that this relay when assuming the right-hand position signifies that the associated push button has been operated and that this has not yet been followed by the insertion of a token in each of the token jacks OTJ and MTJ on the board X (Fig. 2A). It is also readily seen that if either token is inserted in its token jack OTJ or MTJ, board X (Fig. 2A), or ii both of such tokens are inserted before the associated push button PB is operated that the lock relay LR will assume its deenergized position. it is also readily seen that if the mag-stick relay MS is once operated to its abnormal right-hand position that it cannot be restored unless a token is inserted in each of the token jacks OTJ and MT5 for that panel in overlapped relation and that thereafter the lock relay LR will be picked up only if both of these tokens are again removed. It will also be seen that if there is a token .in either or both of the token jacks OTJ or MTJ that depression of the associated push button PB will produce no result. The manner in which these various functions may be used to control indicating lights on the same and on other panels ci the vario-us control and indication boards will be more fully discussed when the operation of the system by one or more operators and monitors is considered hereinafter.

Ground layout-n Fig. 4 has been illustrated a ground plan or ground layout including circular outer stacks WO and EO, the letters W and E signifying west and east, respectively, an oblong central stack CS and intermediate zones WI and EI connecting the outer stacks WO and EO, respectively, to the central stacks CS. As illustrated the central stack CS has associated therewith a holding loop HL with the understanding that this particular type of central stack and holding loop will be used when the invention is practiced in connection with the landing apparatus illustrated in the prior application, Ser. No. 573,876, above referred to. When this landing system is used airplanes are called from the central stack CS one at a time and are instructed to make external holding loop maneuvers in the holding loop HL which may be of various lengt-hs depending upon the length of time to be dissipated before an actual approach toward a runway of the landing eld LF illustrated in Fig. 4 is permitted. These maneuvers may be made in overlapped relationship. It should be understood that other configurations of central stack CS and outer stacks WO and EO, than those illustrated, may be employed if desired. The intermediate zones WI and EI are merely lines for connecting outer stacks to the central stack and preferably do not include any particular ground equipment. rIhe central stack and outer stacks are, however, preferably marked in any suitable way as by radio beacons or aerophares and detected by suitable means such as an automatic direction nnder through the medium of which the pilot may be guided when flying blind as in a heavy overcast of fog or mist.

Push-button circuits-In Fig. 6 of the drawings have been illustrated circuits to show how the push-button partial circuit for controlling a particular mag-stick relay on a particular panel are interlocked with lock relay contacts located on that and other panels of the circuit diagram (Figs. 2A-2E) These interlocked circuits are not shown in their entirety but are merely shown up to and including the token contact 58 (see board X, Fig. 2A).

Referring now to Fig. 6, the contact 58 for board X, outer stack WO (Fig. 6) receives its energy from a terminal (-i-) of a suitable source of direct current, front Contact 61 of the lock relay LR, board Z, intermediate Zone WI, front contact 68 of the lock relay LR for board J, and contact 69 of the lock relay LR of board X, intermediate zone WI, through wire |5, front contact 55 of the lock relay LR of board Z, outer stack WO, front contact 56 of lock relay LR of board J of outer stack WO to the token jack contact 58, from whence the circuit continues and may be traced on Fig. 2A of the drawing through back contact 59 of the lock relay LR of board X, outer stack WO, through the winding of the associated mag-stick relay MS for that board, through pushbutton contact 5| of the push button PB and through contact 50 of the token jack MTJ of this same board X, outer stack WO, to the center connection CC of said suitable source of direct current. A similar circuit may be traced to the token jack contact 58 for outer stack EO and for this reason corresponding contacts of symmetrically related lock relays have been assigned similar reference characters.

A circuit may be traced to the token jack contact 58 of board X, intermediate zone EI (Fig. 6) from the same terminal of said suitable source of current, contacts 6l, 68 and 69 of lock relays for boards Z, J and X, respectively, of zone WI, wire 2|, front contacts 96, 91 and 98 of lock relays for boards X, J and Z, respectively, of the central stack CS (Fig. 6), wire 4|, front contacts I and of lock relays LR of board Z and J respectively for outer stack EO, wire 42, front contacts 55 and 56 of the lock relays LR for boards Z and J, respectively of the intermediate zone EI, to the contact 58 of the operators token OTJ, board X, intermediate Zone EI from whence the tracing of the circuit may be continued on Fig. 2D of the drawings. The contacts for the circuit for token jack contact 58 of board J, intermediate stack EI, which correspond to the contacts I8 and '|I just mentioned will for convenience be designated 88 and 8| and similarly corresponding contacts for the circuit leading to token contact 58 for board Z of intermediate zone EI will for convenience be designated 90 and 9|. For the sake of uniformity and simplicity corresponding contacts for the push-button circuits for the intermediate zone WI will be designated by like reference characters as those for intermediate zone EI except that the contacts 96, 97 and 98 at the central stack for the circuits leading to the intermediate stack EI will be replaced by contacts 86, 81 and 88 respectively. The circuit leading to the token jack contact B for board X of the center stack CS (Fig. 6) may be traced from the terminal (-i) of a suitable source of current, front contacts 82 and 83 of the lock relays LR for boards Z and J respectively for intermediat zone EI, wire 22, front contacts 84 and 85 of the lock relays LR, boards Z and J respectively for intermediate zone WI, wire 2l', front contact 55 of the lock relay LR for board Z, central stack CS, front contact 55 of the lock relay LR for the board J of the central stack to the token jack contact 58 from whence the tracing of the circuit may be continued on Fig. 2C of the drawings as already traced hereinbefore on Fig. 2A. In like manner the contacts for circuits leading to the token jack contact 55 and board J for the central stack CS and corresponding to contacts 82, 83, 84 and 85 just mentioned have for convenience been designated 92, 93, 96 and 95 respectively, and for board Z have been designated |02, |03, |86 and |95, respectively. All of these circuits may be traced in full in Figs. 2A-2E of the drawings.

Green-light circuits-In Fig. '7 of the drawings have been illustrated the circuits for the green lights of Figs. 2A to 2E inclusive. It will be seen that the circuits for the three green lights G for both the operators board and the monitors board for outer stack WO are subject to front contacts of the six lock relays associated with outer stack WO and intermediate Zone WI, The circuit for these green lights may be traced from the terminal (-i-) of a suitable source of current through front contacts |96, l5? and of lock relays LR for boards Z, J and X respectively, of intermediate Zone WI, wire |93, front contacts |59, H0 and il! respectively, of lock relays LR on boards Z, J and X respectively, of outer stack WO (Fig. '7) through these lights G on both the operators and the monitors board to the other terminal of this source of current. Since the green light G for the panel boards X, J and Z of outer stack EO are controlled by similar circuits corresponding contacts have been'designated by like reference characters. It will also be seen that the circuits for the green lights G for the central stack CS (Fig. 7) are subject to front contacts of the nine lock relays located at the central stack CS and intermediate stacks WI and EI and also include Wires 3?, 28 and il. These contacts have been designated |52, H3, la, H5, HG, H1, H8, H9 and |28. It will be seen that the circuit including these contacts l |2- l2l also fed energy to the green lights G for intermediate stacks WI and EI through wires I8 and lll' respectively but that these green lights include contacts of additional lock relays located at outer stacks WO and EO. These additional contacts have been designated |21, |22 and |23. These circuits may also be traced in Figs. .2A-2E of the drawings. All of these circuits in Figs. .2A-2E are obviously duplicated for each of the ten altitudes illustrated in Figs. 1A, 1B and 1C of the drawings.

Operation by one operator and one monitor.- As already pointed out tokens are used as symbols to signify airplanes and the location of such a token on the operators or monitors board siagnies that the airplane it identies is assigned to that route portion or stack altitude. Referring to Figs. 5A and 5B it will be seen that this token constitutes a peg or shank |25 provided with knurled handle |26 over which a perforated card |21 may be pushed, the knurling on the handle |26 serving as a means for preventing the rotation of the card about the token handle. This Vcard is preferably provided with data identifying the airplane and possibly its cargo or destination and/or the pilot of the airplane.

Referring now to Fig. 4 of the drawings let us assume that an airplane numbered |89 is approaching the outer stack WO (Fig. 4) at a particular altitude which is not important to the -present consideration, The operator will prepare ii a token by inscribing the airplane number and other data on the card l2? of a token and will insert the token in one of the three rows of approach jacks shown at the extreme left-hand side of Fig. 3 of the drawings. For the present, consideration will be given only to the board at which the operator is located, that is, it will be assumed that only one operator is present by reason of lovv traiiic density, and let us for this purpose refer to Fig. 3 of the drawings,

In both Figs. 3 and 1A, lB and 1C of the drawings a cross on a push button PB denotes that the push button has been depressed and two tokens have not yet been inserted and removed (see legend push button was depressed). That is, it denotes that a mag-stick relay MS, fifteen of which are shown in Figs. ZA-ZE of the drawings, has been operated to its abnormal right-hand position. A dotted cross in such push button denotes that the mag-stick relay MS was just returned to norinal by both tokens being inserted but not yet removed (see legend mag-stick relay returned to normal). A solid rectangle about a token jack is a symbol to indicate that the operators token is in place (Oprs. token in jack). A dotted rectangle about a token jack signiies that the monitors token is contained in the corresponding token jack (l\/ions token in jack). A solid rectangle and also a dotted rectangle about a token jack. signifies that both an operators token and a monitors token are located in their respective corresponding token jacks (bth tokens in jack).

The monitor who is located at a monitors board, preferably located in a diierent room than that in which the operators board is 1ocated, will also prepare a token identifying this airplane ESQ, and he will insert it in one of the approach token jacks located to the extreme left on his board (Figs. 2li-2E) Let us assume that the airplane i6@ under consideration is approaching the outer stack WO and that the operator decides to have this airplane enter the outer stack WO at a particular altitude. Before he can do this he must preempt, or set aside, a particular altitude for his own use. In order to do this he will depress a push button PB on the panel for the outer stack W0, as for instance indicated by the cross on pushl itton FB for a titude l meaning 19036 feet (Fig. 3). rIhis will result in the lighting of the yeliow light Y for stack WO for the particular altitude under consideration and in the extinguishment oi the green light G as illustrated for altitude iii Fig. 3. t should be understood that all oi the operations presently considered occur at one altitude only but the conditions of the board will be shown at various altitudes in Fig. 3 of the drawings. To accomplish this lighting of the yellow light Y let us assume that the operator depresses the push button PB, board X (Fig. 2A), as a result or which the following circuit for the mag-stick relay MS is closed; beginning at the terminal or a suitable source 0i current iront contacts "l", and 5g of lock relays LR for boards Z, J and X respectively, for the intermediate Zone WI (Fig. 2B) wire iii, iront contacts 55 and 55 of lock relays LR for boards Z and J respectively oi outer stack WO (Fig. 2A) contact or the token jack OTJ, back contact ad of the loci( relay LR (this lock relay having been dropped by the opening of contact 5l or the depressed push button PB), winding oi the inag-sticl relay MS of board X (Fig, 2A) normally vopen eo itact 5l of the push button PB and conta-ct of the monitors token jack MTJ, board X (Fig. 2A) to themid pointCC of lsaid source of current. The completion-of this circuit will cause this mag-stick relay `MS to Vbe operated `to its right-hand position to thereby cause opening or the contactof the mag-stick relay MS so that Ythe closing of baclrccntact 13| of the associated lock VrelayLR-can-notfenergize this red light R and it will'also re'sult inthe fclosure of contact lE32 of vthis mag-stickrelaytolight the yellow light Y (see altitude l-Q for outer Astack WO, Fig. 3). vBy referring 4to Fig. 2A it Will be observed that opening of 'front contacts Ill and liti of lock relay LR 4extinguished the green'lights G not only `for board X of the outerstack WO (Fig, 2A) but also ior board X of the intermediate none WI (Fig, 2B) as is also Vevident :by reference to altitude i9 Aof 3 of the drawings. -Correspending contacts i3! Vand l 32 on the 'three panel boards X, J and Z for outer-stacks WO and EO and for intermediate stacks WI andEI have been designated by like reference characters vvand `contacts for outer stacks'WO and EO corresponding to contact i3@ 'of panel X 4of Fig, 2A have .also been designated by like reference characters.

Referring now to altitude i8 `(Fig. 3) letus assurne that the operator inserts his token |00 in the token jack OTJ for the panel X 'of the outer stack WO (Fig. 2A). As is evidentfrorn thef'circuits of Fig. 2A and as indicated in Fig. 3 this will produce no cha-nge in the indicating lights, Reierring to Fig. 2A such insertion of the token 1lill!) in the token Vjack OTJ, 'board X, `results merely in the opening of the 'contact 52 in the circuit of the lock relay IR which 'is already open and I-'esults in the breaking `or the circuit heretofore traced for 'the mag-stick relay MS and in the preparation of a second circuit for this 'mag-stick relay MS `which is open at another point.

`Referring now to 'altitude 'Il (Fig, 3) let usassurne that l'the operator has instructed the pilot to enter outer stack AWO at aparticular'a'ltitud-e and that `the monitor also inserts his token in .the

token jack MTJ, board X, (Fig. 2A), 'because he' has heard the pilot of the `airplane l'll acknowledge that he will enter the lspeciiied altitude `of stack WO in response to instructions from lthe operator or dispatcher. In this connection it iis desired to again point out that the monitor can only hear the voices of pilots by reason vof the fact that even though the same carrier frequency is employed the operation of a foot pedal in the radio telephone system by the operator which operation is necessary to enable him 'to send a Verbal radio message to the pilot also results in cutting on the monitors radio telephone so that the monitor cannot hear the operator speak. This construction or other construction producing a similar result, is resorted to so that the monitors action of inserting a token in the token jack is entirely in response to what he hears the pilot say and this will result in a check upon the fact that the pilot has properly heard, properly interpreted and properly repeated backthe instructions given to him by the operator. That is, if the pilot reports entering, for instance, a different altitude than the altitude which he was instructed to enter by the operator, the monitor will insert his token in a token jack not corresponding to the token jack in which the operator has inserted his token and lthe desired indications on the lboard will not appear.

Let us now assume that the monitor inserts his token in the token jack MTJ, board X (Fig. 2A), the operator already having inserted his token in the token jack OTJ of the same board (see altitude I1, Fig. 3). A circuit is now closed which may be traced from the terminal of the source of current above mentioned, contact 6| of token jack MTJ, board X (Fig. 2A), contact 58 of token jack OTJ assuming its right-hand position back contact 59 of the lock relay LR for the same board, mag-stick relay MS, contact 62 of this mag-stick relay MS assuming its right-hand position and to the midpoint CC of said source of current. It will be observed that the current flow in this circuit is of opposite polarity from that which flowed in the rst circuit traced for this mag-stick relay MS for which reason this magstick relay MS will be operated back to its original left-hand position. With the contact |323 of the mag-stick relay MS again assuming its lefthand position an energizing circuit for the red light R, board X, outer stack WO (Fig. 2A) will be lighted and the associated yellow light Y will be extinguished by the opening of Contact I 32 as indicated for altitude I1 in Fig. 3 of the drawings. The fact that the mag-stick relay MS has been returned to its normal position is indicated by the dotted cross in push button PB, altitude (Fi-g. 3).

Referring now to altitude IB (Fig. 3) it will be seen that only the monitors token has been removed. This, however, produces no change in the indicating lights for altitude I as compared with the indications displayed for altitude I's' in Fig. 3 of the drawings. In altitude I5 (Fig. 3) only the operators token is shown removed and this also produces no change in the indicating lights as is obvious from the circuits shown in Fig. 2A. If, however, both of these tokens are removed from the token jack of board X (Fig. 2A) the green lights G (see altitude Id of Fig. 3) will again be lighted because the lock relay LR (see board X, Fig. 2A) is again reenerg-ized through its original energizing circuit heretofore traced and including contact 5U of the token jack MTJ, and Contact 52 of the token jack OTJ. This also results in the lighting of the green light G for board X of the intermediate zone WI as is obvious from Fig. '7 of the drawings.

Let us now assume that the operator wishes to allow this airplane |60 to move into the same or a diierent altitude of the central stack CS (Figs. 3 and 4) In order to do so since the green light G for the central stack for a particular altitude is lighted, the green lig-ht for the intermediate zone (see altitude I3, Fig. 3) being extinguished, he will depress the push button PB for the intermediate zone WI for that altitude. This has been illustrated in altitude I3 in Fig. 3 of the drawings. In the event a change in altitude is to be made in flying from the outer stack WO to the central stack CS the operator will be required to depress all of the intervening push buttons for the intermediate zone including those of the exited and entered altitudes and he must receive a lighted yellow light for each of these altitudes before he may instruct a pilot to make such entrance into a new altitude into central stack CS. As the pilot, after having been instructed to make a change in altitude in flying to the central stack, reports arriving at a new altitude both the monitor and the operator move their tokens accordingly from altitude to altitude in the intermediate zone. Also. as the pilot reports entering the central stack the monitor and pilot both move their tokens to the proper altitude in the central stack. Depression of the push button PB for the intermediate zone WI above mentioned causes a circuit to be closed for mag-stick relay MS, board X (Fig. 2B) which may readily be traced in Fig. 6 and Figs.

2A-2D of the drawings by starting at the terminal (-1-), front contacts 69, 68 and 61 of lock relays for boards X, J and Z, intermediate zone EI (Fig. 2D), wire 36, fro-nt contacts 83, 81 and 86 of boards Z, J and X for the central stack CS (Fig. 2C), wire Il, front contacts 'II and 1U of lock relays LR for boards J and Z of outer stack WO (Fig. 2A), wire I2, front contacts 55 and 56 of lock relays LR for boards Z and J, respectively, of the intermediate zone WI (Fig. 2B) contact 58 of the token jack OTJ, board X (Fig, 2B), back contact 59 of the associated lock relay LR, winding of the associated mag-stick relay MS, normally open push button contact 5| of the push button PB`for board X (Fig. 2B), Contact 59 of the monitors token jack MTJ, board X (Fig. 2B), to the midpoint CC of said source of current. The closure of this circuit will cause the mag-stick relay MS for board X of intermediate zone WI (Fig. 2B) to be operated to its right-hand position to thereby light the yellow light Y for the intermediate zone WI as illustrated for altitude |3 in Fig. 3 of the drawings. This operation of the mag-stick relay MS for board X of intermediate zone WI (Fig. 2B) to its right-hand position by the closure of its contact |34 will also cause (-1-) energy to be applied to bus wire 3| to thereby cause the yellow light for board X of the central stack CS to be lighted through contact |32 of the associated mag-stick relay MS assuming its normal lefthand position. For this reason the yellow lamp Y in both the intermediate Zone WI and at the central stack CS (see altitude I3, Fig. 3) will be lighted. 'Ihe red light R for board X at the central stack CS is also lighted through a circuit including the back contact |35 of the lock relay LR for board X, intermediate zone WI (Fig. 2B), bus wire 29 and contact |35 of the mag-stick relay MS, board X, central stack CS, assuming its lefthand position. Although in practice the red light R for the outer stack WO will still be lighted by reason of the presence of a token in the associated token jack, it will remain lighted if such token is removed by reason of a circuit including contact |3| of relay LR, board X (Fig. 2B), bus wire I9 and contact |39 of relay MS, board X (Fig. 2A). These lights Y and R have been shown lighted for altitude I3 (Fig. 3). For convenience corresponding contacts |34 and |35 on panels in Figs, 2B and 2D of the drawings have been designated by like reference characters as have also contacts in Fig. 2C corresponding to contact |36.

Since the operator is confronted with lighted yellow lights (see altitude I3, Fig. 3) at both the intermediate location WI and at the central stack CS he is aware of the fact that this altitude is being held open for his use and he will then instruct the pilot cn airplane |30 to enter the specied altitude. As the pilot reports back these instructions to the operator the operator will insert the token I lll] in the token jack for intermediate zone WI (see altitude I2, Fig. 3), and the monitor who can only hear the pilot and not the operator will in response to this same information insert his token into the corresponding token jack on the monitors board as conventionally illustrated for altitude of Fig. 3 of the drawings. It will be observed that the insertion of the operators token alone produced no change in the indicating lights (altitude I2, Fig. 3) but that as this was followed by the insertion of the monitors token it resulted in the extinguishment of the yellow light for intermediate zone WI as illustrated for altitude in Fig. 3 of the drawings. This extinguishrnent of of its contact 3E extinguishes the yellow light Y, board X, (Fig. 2B). The yellow light Y for board X oi the centrcl stack CS is, however, not extinguished because it is lighted through a circuit includ'ig the contact 38 of the token jack OTJ, intermediate Zone WI, board X (Fig. 2B), bus wire Sii, and contact i3? or" the relay MS, 'board X, central stack (Fig. 2C). The red light R, board X, central stack CS, remains lighted through the medium of back contact AL35 of lock relay LR, board X (Fig. 223), bus wire 211i and contact iEboardX (Fig. 2C).

A token is now present in each of .the token jacks OTJ and MT for board X, intermediate zone WI (Fig. 2B) as illustrated `for altitude il of Fig. 3 of the drawings. This indicating condition on the board (altitude il) assures the operator that the D ot has properly understood the operators inst-.actions is following these instructions.

Let us now assume, for the purpose of discussion only, that the monitor removes his token from the token jack M'IJ. This produces no change in the indicating condition oi the indicating lights, as illustrated for altitude lil as corni pared with altitude ii of Fig. 3 of the drawings. This is true because merely contacts 153 and @i of his token jack MT5 were closed and opened respectively andL they are contained in circuits open at other points. Let us now assume, for the purpose of discussion only, that the monitor has left his token his token jack MTJ but that the operator has removed his token from token jack CTJ, i'...terrnediate Zone W I (Fig. 2B). It will be seen hat this results in the opening of the contac 33 of the token jack OTJ which results in the cxtinguishrnent of the yellow light Y for board X, central stack CS (Fig. 2C) by opening a readily traced circuit. This is indicated for altitude s in Fig. 3 of the drawings. It will be seen by referring to Fig. 3 of the drawings that the red light R for outer stack W0 is shown lighted for each of altitudes 9 to i3, inclusive. is due to the fact that the red light E for panel X, outer stack WO (Fig. 2A) is lighted through a circuit including back Contact itl of the lock relay LR, board X, intermediate Zone WI (Fig. 2B), bug wire i5, and contact or the mag-stick relay MS for board X, outer stack WO (Fig. 2A) assuming its normal left-hand position,

Ii now the monitor also should remove his token from his toiren jack MIJ oi the intermediate zone the indicating lights for outer stack WO, intermediate aone WI, and central stack CS will be returned to their normal green-light-lighted condition as illustrated for altitude 8 in Fig. 3 of the drawings.

In practice the push button .PB for board X, of the central stach CS will loe depressed before the tokens (see altitude il, Fig. 3) are removed from their respective token jacks and such depression oi this push button has been illustrated for altitude l (Fig. 3) it being assumed that the two tokens were removed after this push button der 14 was depressed. vWith thefpush Ibutton PB, board X, central stack'CS (Fig. 2C) depressed a circuit is closed for the associated mag-stick relay MS which 'may be traced in the drawings (Fig. 6) from the .terminal (-1-), or a suitable source of energy, front zcontacts .'82 and -83 of lock relays forrboards J randZ, intermediate Zone WI (Fig. 2D), wire 2'2, front 'contacts 61! fand 85 of lock relays LR for boards yZ and J, intermediate Zone WI (Fig. `2B) `wire .27, front `contacts 55 and 56 of lock relays fior vboards Z Aand J of central lstack CS (FigC'), acontafct of ithe token jack OTJ, board X (Fig. 12C), back contact 59 ofthe 4associated .lock :relay LR, winding 'of the associated mag-stick relay MS, A:push button Contact 5| of the `-associated push button PB, 'and 'Contact 5i) or" the :associated-monitors token jack MTJ to the midpoint 'CC tof Asaid source of current. The completion of this circuit causes Vthe mag-stick relay MS, board X, central stack CS (Fig. 2C) to he operated to its .right-hand position to thereby light its yellow light Y-as illustrated for altitude l, Fig of .the drawings. It also causes the green light tGr for board X, `interrnruiiate "to Zone WI to foe 'extinguished by the opening of .iront Contact lil' (Figs. .2C and 7) of :lock relay LR, board X, central stack CS. 11n practice this green light was, however, already extinguished by tokens in the token {ja-cles =of fthe corresponding intermediate zone. If now 'the pilot .reports arriving at the proper :altitude yin the central stack :CS the operator inserts his token |1313 in the token jack O'IJ, board X, vcentral stack rCS (Fig. 2C) no parti'cu'lar 'function (rsee altitude 6, Fig. 3) will be accomplished in that :the circuits made and broken are iopen at yotherpeints, so that the yellow light Y, 'board centra-l stack, remains lighted as illustrated .for altitude 6 in Fig. 3 of .the drawings. If, however, the monitor also hears this report of the pilot v'andiinserts his token l in his token jack MTJ, board X, 'central stack CS (Fig. 2C), the associated mag-stick relay MS will ybe returned .toits .normal .left-hand position and this by the opening of contact ISZ will result in the f extinguis'hment vof the associated yellow light Y and the lighting of the associated red light R (see 'altitude 5, Fig. 3). This red light R is lighted through back contact L35 of lock relay LR, board X, central stack 4CS 'and through the contact H36 of .the associated .mag-stick relay MS. This condition of the apparatus has been illustrated for altitude 5 in Fig. 3 Iof the drawings. The green light G at both of the intermediate stacks are still extinguishedby reason of 'open front contact ll-I of relay'LR, board X, control stack CS (see Fig. 7). The removal of the monitors token alone, as Ain 4response to the pilot reporting descending to a lower altitude, produces no particular result as isobvious from the circuits described andas has been conventionally shown for altitude 4 in Fig. 3 of the-drawings. Likewise, the removal of the operators token alone, in response to such report, produces no change'in the .indications as indicated for 'altitude 3 in Fig. `3 of the drawings. If, however, both of these token are removed the red light R vfor board X, central stack CS (Fig. 2C) will be extinguished andthe associated green light will be lighted as will also the green lights for the intermediate zones WI and EI, all as illustracted for altitude -2 in Fig. 3 of the drawings.

In the foregoing operation the indications given on boards J andiZ have been substantially entirely disregarded, because it was assumed that only the opera-.tor X waspresent -and that the indications on 'boards J .and Z were of no importance.

`every other outer stack, suoli as stack EO.

We will presently consider the operation of the system when three operators conveniently called operators X, J and Z Will all be present each one operating his own board similarly designated the three operators' board being preferably located side by side in one room and the three monitors boards being preferably located side by Side in another room. entirely separated from the operators room. This separation of the operators apparatus and the monitors apparatus is deemed necessary to obtain proper safeguards in carrying out a double check of the pilots repeat-back acknowledgment of instructions received by him from the operator. Although the employment of a monitor and the apparatus necessary to provide such control is for safety reasons deemed desirable it should be understood that the monitors apparatus and the functions performed thereby may be entirely omitted from the system, if desired.

Operation by three operators and three mom'- trs.-It should be understood that the circuits and apparatus illustrated in Figs. ZA-ZE is that apparatus necessary and associated with a single altitude on the operators board. and the monitors board. In practicing the invention it is proposed that at least ten different altitudes will be employed, these altitudes being preferably separated 1000 feet for which reason the control boards X, J and Z illustrated in Figs. 1A, 1B and 1C respectively, illustrate ten different altitudes l to ill inclusive, each number signifying 100() feet. The circuits for each of these altitudes are identical. In addition there is provided a ground altitude GR, this ground altitude being employed to show which airplane has been granted the privilege of actually landing on the landing strip on the airfield and to show the progress of such landing. Also, although in Fig. 4 only two outer stacls WO and EO. and their associated intermediate zones WI and EI have been illustrated. it should be understood that any desired number of outer stacks and associated intermediate zones may be employed.

Let us now assume that there are three operators X, J and Z at the operators boards X, J and Z illustrated in Figs. 1A, 1B respectively, and that each operator is provided with a monitor who sits at an associated monitcrs board which is identical to the operators board except that the green lights at intermediate locations WI EI have been omitted and that no push buttons are provided.

Referring now to board X (Fig. 1A) let us assume that the operator X Wishes to bring airplane into the outer stack WO. It is of course understood that the operators J or or both at boards J or Z (Figs. 1B 1C) be contemplating landing airplane in outer stacl; WO at the same time, in that each of the t; rec operators has the same privilege of u all altitudes in this outer stack WO as well as in In order to prevent two operators attempting the entry of an airplane in this outer stack WO at the same time push buttons have been pro vided to enable an operator to pre-empt, or set aside for his own use a particular altitude in a particular outer stack. These push buttons and the associated apparatus function so that if two operators attempt simultaneously to pre-empt to themselves the saine altitude neither of these operators will obtain the right to enter such altitude. This is brought about by reason of the fact that one lock relay LR must be in a deenergized position before the associated magstick relay can be operated and the corresponding mag-stick rela-ys on the other two boards cannot be operated unless such lool; relay is in an energized condition and furthermore each operated mag-stick relay holds its associated lock relay deenergized. In other words, only one of three corresponding mag-stick relays can be operated at any one time.

Let us now assume that the operator depresses the push button PB in outer stack WO for a particular altitude, altitude il for instance. lt wiil be understood that for reasons heretofore given that suc depression of the push button will cause his green light G to be extinguished and his yellou7 light Y to be lighted for outer stack WO (Eig. 1A). Also, that the corresponding green 'ight C- for the intermediate zone WI will be extinguished all for reasons heretofore given (see altitude S). In addition to this result on board X the c rresponding green light G for outer stack WO on boards J and Z (Figs. 1B and 1C) will be extinguished and their associated red lights R will be lighted as illustrated for altitudes t in Figs. lA, 1B and 1C of the drawings. The green lights G for intermediate Zone W'I will, however, be extinguished on all three boards (see altitude gs. lA, 1B and 1C). Let us now observe how and through what means these green lights for outer ack WO on boards J and Z will be extinguished and their associated red lights R lighted. Referring to Fig. 2A of the drawings it will be observed that the depression of the push button PB, for .Joard X (Fig. 2A) will result in operation of ciated mag-stick relay MS to its dotted p( v"Ellis operation of this relay -ation. MS will cause the associated green light G to be extingu. hed and the associated yellow light Y to be lighted. A.iso, the opening of contact 136 of this relay prevents the red light R on panel X being lighted through a circuit including the back contact l-i or" the associated lock relay but this contact Ei may feed energy over the bus wire I9 to the red lights R on panel boards J and Z for outer stack WO (Fig. 2A) and these red lights R have been illustrated lighted at altitude 8 of these boards J and Z (Figs. 1B and 1C) Also, the opening of contact H l and E'Zi outer stack WO of the associated lock relay LR will cause the green lights G to be extinguished at each of points WO and WI, on all three boards (see Figs. 1A, 1B and 1C and the circuit illustrated in Fig. 7).

Referring now to altitude '.1 (Fig. 1A) the insertion of the operators token and the monitors token in their respective token jacks on boards X (Fig. 2A) for reasons heretofore given does not change the lighting of the indicating lights on either the operators or the monitors boards J and Z (Figs. 1B. 1C, 2A and 2B) but merely extinguishes the yellow light for operator-s and monitors board X and lights the associated red light for the operators and monitors board X as illustrated at altitude l, Fig. 1A, and as shown partly in Fig. 2A all for reasons heretofore explained. .Since the monitors indicating lights are connected in multiple With the opcrators lights these monitors lights need not be specifically mentioned hereinafter.

Let us now assume that tl e operator of board X will try to prei-empt for his own use the altitude E5 in the central stack CS. He will observe that the green light G for the central stack CS is lighted (see altitude Fig. 1A) which informs him that there is a possibility of setting aside this altitude' (see altitude 6) hisc'v/n use. rihel oper the central stack for of the boards J and Z as illustratedin Figs. iB

and 1 of the drawings.

Let usnow observe howthis green light on the boards J andy Z was extinguished and its associated red` light was` lighted. Referring to Figs. 2B, 2C and 2D of the drawingsit will be observed that dropping of the lool; relay LR, board X, intermediate zone WI (Fig. 2B) by the opening oi its contact lil not only causes the green lights G on eachof the boards X', J and Z for the interined-latev zonel WI to be extinguished but also causes the three greenlights for the boards X, J and Z oi the-central stack and for the intermediate zone El to be extinguished, all as is clearly. evident from the` circuits illustrated in Figs. Z, 21C, 2D and l of the drawings. Attention is particularly directed to Fig. 7 oi thev drawings which shows, in condensed forni, the circuits for lightingthe various green lights G.

Referring again to Figs. l; 1B and 1C of the drawingsit Will be observed that the insertion oi theV operators token and the monitors token in their' respectit'fe token jacks for intermediate Zone' WI", as' conventionally shown for altitude (Fig. lA) produces no change in the indications given" on boards'J and Zv (Figs. 1B and 1C) but' doesv extinguish the yellow light Yl for the in'- termediate zone WI on board X (see altitude 5, Fig. 1A); This extinguishhie'nt' ofl the' yellow light Y for intermediate zone' 'Wi is brought about by the opening' oi" theconta'ct i132' of the associated magstick relay MSA (Fig. 2B). The yellow light Y for the central stack is not changed because even though contact board Xl (Fig. 2B) isopenedV the bus Wire 3i is now energized through contact 33 of tokenl jackO-TJ; board'X (Fig. 2B).

eferring now to altitude l (FigA 1A) it will be observed that the removal of the monitors token alone produces noichange of' indicationon board X (Fig. 1K) but' by referring loalttude (1B-'it Willv be observed that the removal of the operators token with' the inonitors token retained in his token jaclr.v willi cause the yellow light at the' central sta-cl:` oniboardlX-to be extinguished but produces no change of indication on theboar'ds J and Z (Figs. 1B and 1C). Tiis extinguishment of theyellow light Y" on thecentral stack CS; board'Xlaltitud'e 3; Fig. 1A) isI due tothe fact that the tolien jack contact E38-Qv board X,

intermediat'ezone WI' ('ig. 2B) has been opened and has resulted ini the removalof energy from bus Wirel, which hadibee'n feeding energy tothe yellow light Y oni board" X ofj the central stack CS (Figi 2C) through the-contact it?Y or its associated' mag-stickrelayf MSI It is" this openingv oilthiscontact Hi8 Which'resulted'from the removal of the operators token but which does noti result from the removal of'thenlonitors token that causes this yellow light Y to be extinf ,or will therefore inA practicing the invention depress push button BBk Referring now to boards J and i8- guished When one of the tokens is removed but not to be extinguished when the othei` token is removed?. The removal oi' both tokens; howevers resultsl inA the extinguishnient of theV redlighti on" outer stack- WO on eachA oil the board'sX, Jl a-nd.-v Z? and results in the lightingl oiV theassociated green'- light G (see altitude i, Figs.; 1A; 1B? and- 1G);

In practice these tolens will!r notV be r'errioled'` iront the intermediate panel WI until the operator hasl depressed the push button PBj for the central stack CS.l Such depression' ofv the' push button for the saine or a diiere'nt altitudeof ce stack CS a's has been conventionally illustrated bythe solid crossv in the'push' button' PB for altitude 2, board central' stack" CS' (Fig. la). In the present instance the" yellow' light for the central stack is lighteduthroughf af circuit including the Contact 32 of the associatedmag-stickA relay MS assuming itsright--hand dottedposition (see board Z2, central stack GS; 1

2C). This condition has been conventionally illustrated by the lighting o1 the yellow, light Y following the depression of Ithe push buttonf PB at altitude 2`, Fig. 1A' of the drawings, -Ifnow iii response to the pilots. report thatv hehas entered the stack CS both the operator and the monitor insertI their tokens in their respectiveY token jacks" as illustrated for altitude l,. central Stack CS (Fig. AY the yellow'A light ,ills-ti idlento'ned' is 6X't'ii'iguishedA and the associated redlight is iigiited. No enangsof indications; tri boards'J andZ', however, take plac'e (see altitude l ascempareci witnaititue'z, Figs. 1B`a`rid 1C);` Referring to Fig'. 2C this- Gh'aif OK 'd'cati for the` central stackV panel board X is, brought about by the're't'u'rn oi the" i'r'i'ag-sticlrelay'M'S', board centraistack CS (Fig. zoitoitsnenrai position; causing' this yellow light" Y t` a'gar be" connected to-bus Wire fil whichy bus" Wirew is new deenergize'd,` the associated red( light beingh lighted by the reclosureof the"ContactI `of this mag-stick relay MS. it being under.stjo'od-` thati the backc'ont-'act l'irofthe associated; loclr relay LR'is still closed andeenergi'zesbus'-Wire 29. The" red-lights R5 for? this altitude l5 the Other l'fV'wfC'1 bo'ardsJ and-Z A'nsling maintaine'dl-ighted 'thru'g'l' liis same back-contacti3d-'because the' rlag-sticl relays NIS folf'bbarcsk aridiZi'Olalllflldev l r# main in' their nor-final: position arid'y these? red lights are all` connected tu this sari'ie' for which reason all? three of the r-e'd-l'ightsforA altitudey lg central stack GS; boards XMI and? ZF arelighted;

-r" now, both ofl the tblens are' removed from'7 their respective tollen jacks the locle relay LR for board X; centralstack-CS v (lug-2(5)l Willbe .be observed that allo" the circuits* shown therein are syin'rnetriea1l when considered fromt'he exi' treme two lenesforltiie drawings toward the-:meets or central stackCSl Fromthis lconsideration#itr isobv-ious `that-the operationsjiustdescribedwhich 'started with outer stack WO` and ended with` 

